Summary of Modules 5 and 6 - PowerPoint Presentation

1. Summary of Modules 5 and 6Lydia R. Leonardo, DrPHBio 133 Lec

2. Module 5: Cleavage and Blastulation

3. CleavageRapid cell division resulting in a single fertilized egg into a multicellular embryo known as the blastula or blastodisc.Cell division may be slowed down by moderate amount of yolk resulting in formation of unequal size of cells.

4. The EggAnimal pole – consisting the upper hemisphere of the egg (dark stained area)Vegetal pole- consisting the lower hemisphere of the egg (light stained area)Unfertilized eggs – unsegmented and enclosed tightly by transparent jelly coats > basically for protection

5. Classification of EggsBased on amount of yolk Megalecithal – Huge amount of yolk present Ex. Birds, reptiles, bony fishMesolecithal- Moderate amount of yolk present Ex. AmphibiansMicrolecithal –Very little amount of yolk present Ex. Amphioxus, mammals

6. Classification of EggsBased on distribution of yolkIsolecithal- yolk is evenly distributed all throughout the egg. Ex. Sea urchins, Amphioxus, HumansTelolecithal- yolk is distributed in a gradient Ex. Reptiles, fish, birds, amphibiansCentrolecithal- yolk is concentrated in the center of the egg. Ex. Mostly arthropods

7. Cleavage TypesHoloblastic- total cleavage formation. Entire egg is divided thus resulting a successive blastomeres.Equal = microlecithal eggs, equal sizes of divided cellsUnequal = mesolecithal eggs, ventral region of the egg is larger than on the dorsal region.Meroblastic – there is a small area on the animal pole that division only occurs resulting -> blastodisc

8. Cleavage PatternsRadial cleavage = symmetrical cleavages at first division ( echinoderms and chordates) -> deuterostomesSpiral cleavage = cleavages are rotated from the first division (annelids, mollusca, some invertebrates) -> protostomesThe end product of cleavage is the blastula-> consists of (micro- and meso-)

9. Stages and Parts of Cleavage in Frog’s EmbryoFrog’s Early Blastula x.s.Animal poleBlastocoelGrey CrescentMacromeresVegetal PoleEndodermVitelline membraneMicromeresEctodermPigmented Cortex

10. Avian Blastula

11. Mammalian Blastula

12. Attachment in the Uterus

13. Module 6: Gastrulation

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15. Gastrulation Migration of cells leading to the formation of a 3-germ layered embryon, the gastrula A highly orchestrated movements of cells that establish the basic body plan. Cells are brought into new positions, allowing inductive interactions.

16. “It is not birth, marriage, or death, but GASTRULATION, which is truly the most important time in your life” –Lewis Wolpert

17. Morphogenetic Movements Invagination Inpocketing of cell sheet into the embryo InvolutionInturning of the cell sheet over the basal surface of the outer layer IngressionMigration of individual cells into the embryo

18. Morphogenetic Movements DelaminationSplitting or migration of one sheet into two sheets EpibolyThe expansion of one cell sheet over the other cells

19. 1. INVAGINATION 2. INVOLUTION 3. EPIBOLYinvagination  Pharyngeal endoderm involution  roof of archenteronAmphibian Gastrulation

20. Steps in Gastrulation1. INVAGINATIONCells begin to invaginate at the dorsal lip of the blastopore

21. Steps in Gastrulation2. INVOLUTION Cells move over the dorsal lip and begin to move into the blastopore

22. Steps in Gastrulation3. EPIBOLY Blastocoel collapses and is replaced by the archenteron. Formation of 3 germ layers (ectoderm, mesoderm endoderm)* Yolk plug is internalized* Completion bridge – thin layer between the forming gastrocoel and the diminishing blastocoel

23. Frog Gastrulation

24. Early Gastrula

25. Late Gastrula

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28. Thank you.